U.S. patent number 5,105,558 [Application Number 07/684,064] was granted by the patent office on 1992-04-21 for apparatus and process for drying cellulosic and textile substances with superheated steam.
Invention is credited to Donald P. Curry.
United States Patent |
5,105,558 |
Curry |
April 21, 1992 |
Apparatus and process for drying cellulosic and textile substances
with superheated steam
Abstract
Apparatus and process for the removal of moisture from
cellulosic products such as pulp, paper and molded articles, and
textile products, wherein gaseous water (steam) in a superheated
state is utilized in an energy efficient manner as the drying
medium. The apparatus is an enclosed system the operates
continuously to dry the products while preventing air contamination
from entering the apparatus and thereby reducing its drying
efficiency. The apparatus comprises a plurality of drying sections
wherein the number of drying sections connected in series is a
function of the type and volume of the product or products to be
dried. Within each drying section there exists indirect steam
heating means, steam-recirculation means, an individual
steam-supply chamber, and sections of a steam-return chamber which
is open and common to all drying sections in the series. Air/vapor
lock chambers at both ends of the apparatus prevent air incursion
into the drying sections and at the same time permit a slight
amount of excess steam from within the drying sections to vent out
of the apparatus. Steam-condensing means located substantially
entirely within the drying sections of the apparatus is utilized to
recover a large percentage of the energy of the heat of
vaporization. The process described herein involves conveying the
products into the apparatus and directing the drying steam from the
steam-supply chamber onto the products and drawing the spent steam
away from the products. The spent steam, which comprises the drying
steam in combination with the steam formed by the liberation of
moisture from the products, is partially recirculated past the
indirect heating means and back into the steam-supply chamber, and
partially condensed by the internal steam-condensing means. The
velocity at which the drying steam is directed onto the products
can be varied in accordance with the type of product to be
dried.
Inventors: |
Curry; Donald P. (South
Portland, ME) |
Family
ID: |
24746560 |
Appl.
No.: |
07/684,064 |
Filed: |
March 28, 1991 |
Current U.S.
Class: |
34/449; 34/213;
34/77; 34/219 |
Current CPC
Class: |
F26B
21/004 (20130101); F26B 23/002 (20130101); F26B
21/02 (20130101); Y02P 70/10 (20151101); Y02P
70/405 (20151101) |
Current International
Class: |
F26B
23/00 (20060101); F26B 21/00 (20060101); F26B
21/02 (20060101); F26B 003/00 () |
Field of
Search: |
;34/155,156,219,218,75,78,17,60,23,37,86,209,212,210,213
;162/202,204,207,289 |
Primary Examiner: Bennett; Henry A.
Assistant Examiner: Gromada; Denise L. F.
Attorney, Agent or Firm: Bohan; Thomas L.
Claims
I claim:
1. Apparatus for the removal of water from cellulosic and textile
products, wherein said water is removed by steam drying means, said
apparatus comprising:
a. product-conveying means for receiving and conveying said
products into and through said apparatus, and for transferring said
products away from said apparatus;
b. a wet-end steam drying section, a plurality of intermediate
steam drying sections, and a dry-end steam drying section, wherein
said wet-end steam drying section, said intermediate steam drying
sections, and said dry-end steam drying section each comprise:
i. steam-recirculation means for directing drying steam onto said
products and drawing spent steam away from said products;
ii. indirect heating means for heating said drying steam, wherein
said indirect heating means is positioned in close proximity to
said steam-recirculation means;
iii. an isolated steam-supply chamber, wherein said
steam-recirculation means directs said drying steam past said
indirect heating means and into said steam-supply chamber, and
wherein said drying steam passes from said steam-supply chamber
onto said products; and
iv. an open steam-return chamber, wherein said product-conveying
means passes through said open steam-return chamber, wherein said
steam-recirculation means draws said spent steam from said products
and into an upper portion of said open steam-return chamber, and
wherein said open steam-return chamber is common to said wet-end
steam drying section, to said intermediate steam drying sections,
and to said dry-end steam drying section;
c. internal steam-condensing means for condensing an excess of said
spent steam, wherein said excess of said spent steam within said
open steam-return chamber comes in contact with, and is condensed
by, said internal condensing means, and wherein said internal
steam-condensing means is positioned essentially entirely within
said open steam-return chamber;
d. a wet-end air/vapor lock chamber for preventing the ingress of
air into said wet-end steam drying section, wherein said
product-conveying means first passes through said wet-end air/vapor
lock chamber and into said wet-end drying section, and wherein air
entering said apparatus with said product-conveying means and spent
steam exiting from said wet-end steam drying section are drawn into
and exhausted from said wet-end air/vapor lock chamber;
e. a dry-end air/vapor lock chamber for preventing the ingress of
air into said dry-end steam drying section, wherein said
product-conveying means passes from said dry-end steam drying
section and into said dry-end air/vapor lock chamber, and wherein
air entering said apparatus through said product-conveying means
and spent steam exiting from said dry-end steam drying section are
drawn into and exhausted from said dry-end air/vapor lock chamber;
and
f. insulation means for maintaining the temperature level within
said apparatus, wherein said insulation means is affixed between
interior walls of said apparatus and exterior walls of said
apparatus.
2. The apparatus as claimed in claim 1 wherein said interior walls
of said apparatus are sealed to prevent steam within the interior
of said apparatus from contacting said insulation means.
3. The apparatus as claimed in claim 1 wherein said
steam-recirculation means is a variable-speed plug fan.
4. The apparatus as claimed in claim 3 wherein said variable-speed
plug fan further comprises a plug fan shaft seal, wherein said plug
fan shaft seal limits the ingress of air from about a shaft of said
variable-speed plug fan and into said drying sections to about 1 to
3 cubic feet per minute.
5. The apparatus as claimed in claim 1 wherein said indirect
heating means is a gas-fired indirect heater.
6. The apparatus as claimed in claim 1 wherein said isolated
steam-supply chamber further comprises a plurality of steam-supply
ducts, wherein said drying steam enters each of said plurality of
steam-supply ducts, wherein said steam-supply ducts are positioned
above and below said product-conveying means, and wherein said
drying steam passes through said steam-supply ducts and onto said
products.
7. The apparatus as claimed in claim 6 wherein one-half of said
plurality of steam-supply ducts are top steam-supply ducts and the
other one-half of said plurality of steam-supply ducts are bottom
steam-supply ducts, wherein said top steam-supply ducts are
positioned above said product-conveying means and said bottom
steam-supply ducts are positioned below said product-conveying
means.
8. The apparatus as claimed in claim 6 wherein said steam-supply
ducts comprise a plurality of steam-supply nozzles, wherein said
steam-supply nozzles are affixed to supply duct faces of said
steam-supply ducts and wherein said steam-supply nozzles direct
said drying steam onto said products.
9. The apparatus as claimed in claim 8 wherein said steam-supply
nozzles comprise openings 2-inches.times.2-inches in
cross-section.
10. The apparatus as claimed in claim 6 wherein said steam-supply
ducts comprise supply duct faces, wherein said drying steam is
directed through said supply duct faces onto said products, and
wherein said supply duct faces are substantially entirely open.
11. The apparatus as claimed in claim 1 wherein said internal
steam-condensing means comprises:
a. a cooling water tube located substantially entirely within said
apparatus, wherein said cooling water tube runs along a floor of
said open steam-return chamber, and wherein cooling water within
said cooling water tube is introduced to each of said steam drying
sections;
b. means for conveying said cooling water from a cooling water
source into said cooling water tube;
c. a sealed cooling tube ingress orifice located in a floor of said
open steam-return chamber of said wet-end steam drying section,
wherein said cooling water tube enters said open steam-return
chamber through said sealed cooling tube ingress orifice;
d. a sealed cooling tube egress orifice located in a floor of said
open steam-return chamber of said dry-end steam drying section,
wherein said cooling water tube exits said open steam-return
chamber through said sealed cooling tube egress orifice;
e. means for conveying said cooling water away from said cooling
water tube;
f. a control valve for regulating the volume of cooling water
passing through said cooling water tube, wherein said control valve
is positioned on a portion of said cooling water tube which is
external to said apparatus; and
g. means for removing steam condensate from said apparatus, wherein
said steam condensate is formed by the condensation of said excess
of spent steam contacting said cooling water tube.
12. The apparatus as claimed in claim 11 wherein said means for
removing steam condensate comprises a condensate tray, wherein
condensed steam located on said cooling water tube drains into said
condensate tray, wherein said condensate tray is positioned under
said cooling water tube and wherein a condensate drain of said
condensate tray transfers said condensed steam away from said
apparatus.
13. The apparatus as claimed in claim 1 wherein said wet-end
air/vapor lock chamber comprises:
a. a wet-end product entry port through which said
product-conveying means enters said wet-end air/vapor lock
chamber;
b. a wet-end conveyor return port through which said
product-conveying means exits said wet-end air/vapor lock
chamber;
c. a wet-end dewpoint sensing device, wherein said wet-end dewpoint
sensing device is affixed to an inside wall of said wet-end
air/vapor lock chamber and wherein said wet-end dewpoint sensing
device measures the dewpoint within said wet-end air/vapor lock
chamber and transmits the dewpoint measurement to a wet-end
dewpoint controller device, wherein said wet-end dewpoint
controller device is affixed to an outside wall of said wet-end
air/vapor lock chamber;
d. a wet-end steam drying section entry port through which said
product-conveying means enters said open steam-return chamber of
said wet-end steam drying section;
e. a wet-end steam drying section conveyor return port through
which said product-conveying means exits said open steam-return
chamber of said wet-end drying section;
f. a wet-end exhaust fan, wherein said wet-end exhaust fan draws
air from the atmosphere surrounding said apparatus through said
wet-end product entry port and said wet-end conveyor return port
into said wet-end air/vapor lock chamber, wherein said wet-end
exhaust fan draws spent steam from said open steam-return chamber
of said wet-end drying section through said wet-end steam drying
section entry port and said wet-end drying section conveyor return
port into said wet-end air/vapor chamber; and
g. wet-end exhaust control means for regulating the rate at which
said air and said spent steam are exhausted from said wet-end
air/vapor lock chamber by said wet-end exhaust fan.
14. The apparatus as claimed in claim 13 wherein said dry-end
air/vapor lock chamber comprises:
a. a dry-end steam drying section exit port through which said
product-conveying means enters said dry-end air/vapor lock
chamber;
b. a dry-end steam drying section drying section conveyor return
port through which said product-conveying means exits said open
steam-return chamber of said dry-end steam drying section;
c. a dry-end dewpoint sensing device, wherein said dry-end dewpoint
sensing device is affixed to an inside wall of said dry-end
air/vapor lock chamber and wherein said dry-end dewpoint sensing
device measures the dewpoint within said dry-end air/vapor lock
chamber and transmits the dewpoint measurement to a dry-end
dewpoint controller device, wherein said dry-end dewpoint
controller device is affixed to an outside wall of said dry-end
air/vapor lock chamber;
d. a dry-end product exit port through which said product-conveying
means exits said dry-end air/vapor lock chamber;
e. a dry-end conveyor return port through which said
product-conveying means exits said dry-end air/vapor lock chamber;
and
f. a dry-end exhaust fan, wherein said dry-end exhaust fan draws
air from the atomosphere surrounding said apparatus through said
dry-end product exit port and said dry-end conveyor return port
into said dry-end air/vapor lock chamber, wherein said dry-end
exhaust fan draws spent steam from said open steam-return chamber
of said dry-end steam drying section through said dry-end steam
drying section exit port and said dry-end steam drying section
conveyor return port into said dry-end air/vapor lock chamber;
and
g. dry-end exhaust control means for regulating the rate at which
said air and said spent steam are exhausted from said dry-end
air/vapor lock chamber by said dry-end exhaust fan.
15. The apparatus as claimed in claim 14 wherein said wet-end
dewpoint controller device and said dry-end dewpoint controller
device control the opening and closing of a control valve of said
internal steam-condensing means.
16. The apparatus as claimed in claim 2 wherein said interior walls
of said apparatus are welded together.
17. Apparatus for the removal of water from cellulosic and textile
products, wherein said water is removed by steam drying means, said
apparatus comprising:
a. product-conveying means for receiving and conveying said
products into and through said apparatus, and for transferring said
products away from said apparatus;
b. a wet-end steam drying section, a plurality of intermediate
steam drying sections, and a dry-end steam drying section, wherein
said wet-end steam drying section, said intermediate steam drying
sections, and said dry-end steam drying section each comprise:
i. steam-recirculation means for directing said drying steam onto
said products and drawing spent steam away from said products;
ii. indirect heating means for heating said drying steam, wherein
said indirect heating means is positioned in close proximity to
said steam-recirculation means;
iii. an isolated steam-supply chamber comprising a plurality of
steam-supply ducts, wherein said steam-recirculation means directs
said drying steam past said indirect heating means and into said
steam-supply chamber, wherein said drying steam enters each of said
plurality of steam-supply ducts, and wherein said steam-supply
ducts are positioned above and below said product-conveying means;
and
iv. an open steam-return chamber, wherein said product-conveying
means passes through said open steam-return chamber, wherein said
drying steam from said steam-supply ducts drys said products,
wherein said steam-recirculation means draws said spent steam from
said products and into an upper portion of said open steam-return
chamber, and wherein said open steam-return chamber is common to
said wet-steam drying section, to said intermediate steam drying
sections, and to said dry-end steam drying section;
c. internal stream-condensing means for condensing an excess of
said spent steam, wherein said excess of said spent steam within
said open steam-return chamber comes in contact with, and is
condensed by, said internal condensing means, said internal
steam-condensing means comprising:
i. a cooling water tube located substantially entirely within said
apparatus, wherein said cooling water tube runs along a floor of
said open steam-return chamber, wherein cooling water within said
cooling water tube is introduced to each of said steam drying
sections;
ii. means for conveying said cooling water into and out of said
cooling water tube;
iii. a sealed cooling tube ingress orifice located in a floor of
said open steam-return chamber of said wet-end steam drying
section, wherein said cooling water tube enters said open
steam-return chamber through said sealed cooling tube ingress
orifice;
iv. a sealed cooling tube egress orifice located in a floor of said
open steam-return chamber of said dry-end steam drying section,
wherein said cooling water tube exits said open steam-return
chamber through said sealed cooling tube egress orifice;
v. a control valve for regulating the volume of cooling water
passing through said cooling water tube, wherein said control valve
is positioned on a portion of said cooling water tube which is
external to said apparatus; and
vi. a condensate tray, wherein condensed steam located on said
cooling water tube drains into said condensate tray, wherein said
condensate tray is positioned under said cooling water tube and
wherein a condensate drain of said condensate tray transfers said
condensed steam away from said apparatus;
a wet-end air/vapor lock chamber for preventing the ingress of air
into said wet-end steam drying section, wherein said
product-conveying means first passes through said wet-end air/vapor
lock chamber and into said wet-end drying section, said wet-end
air/lock chamber comprising:
i. a wet-end product entry port through which said
product-conveying means enters said wet-end air/vapor lock
chamber;
ii. a wet-end conveyor return port through which said
product-conveying means exits said wet-end air/vapor lock
chamber;
iii. a wet-end dewpoint sensing device, wherein said wet-end
dewpoint sensing device is affixed to an inside wall of said
wet-end air/vapor lock chamber and wherein said wet-end dewpoint
sensing device measures the dewpoint within said wet-end air/vapor
lock chamber and transmits the dewpoint measurement to a wet-end
dewpoint controller device, wherein said wet-end dewpoint
controller device controls the opening and closing of said control
valve, and wherein said wet-end dewpoint controller device is
affixed to an outside wall of said wet-end air/vapor lock
chamber;
iv. a wet-end steam drying section entry port through which said
product-conveying means enters said open steam-return chamber of
said wet-end steam drying section;
v. a wet-end steam drying section conveyor return port through
which said product-conveying means exits said open steam-return
chamber of said wet-end drying section;
vi. a wet-end exhaust fan, wherein said wet-end exhaust fan draws
air from the atmosphere surrounding said apparatus through said
wet-end product entry port and said wet-end conveyor return port
into said wet-end air/vapor lock chamber, wherein said wet-end
exhaust fan draws spent steam from said open steam-return chamber
of said wet-end drying section through said wet-end steam drying
section entry port and said wet-end steam drying section conveyor
return port into said wet-end air/vapor lock chamber; and
vii. wet-end exhaust control means for regulating the rate at which
said air and said spent steam are exhausted from said wet-end
air/vapor lock chamber via a wet-end exhaust stack;
e. a dry-end air/vapor lock chamber for preventing the ingress of
air into said dry-end steam drying section, wherein said
product-conveying means passes from said dry-end steam drying
section into said dry-end air/vapor lock chamber, said dry-end
aie/vapor lock chamber comprising:
i. a dry-end steam drying section exit port through which said
product-conveying means enters said dry-end air/vapor lock
chamber;
ii. a dry-end steam drying conveyor return port through which said
product-conveying means exits said open steam-return chamber of
said dry-end steam drying section;
iii. a dry-end dewpoint sensing device, wherein said dry-end
dewpoint sensing device is affixed to an inside wall of said
dry-end air/vapor lock chamber and wherein said dry-end dewpoint
sensing device measures the dewpoint within said dry-end air/vapor
lock chamber and transmits the dewpoint measurement to said dry-end
dewpoint controller devcie, wherein said dry-end dewpoint
controller device controls the opening and closing of said control
valve, and wherein said dry-end dewpoint controller device is
affixed to an outside wall of said dry-end air/vapor lock
chamber;
iv. a dry-end product exit port through which said
product-conveying means exits said dry-end air/vapor lock
chamber;
v. a dry-end conveyor return port through which said
product-conveying means exits said dry-end air/vapor lock chamber;
and
vi. a dry-end exhaust fan, wherein said dry-end exhaust fan draws
air from the atmosphere surrounding said apparatus through said
dry-end product exit port and said dry-end conveyor return port
into said dry-end air/vapor lock chamber, wherein said dry-end
exhaust fan draws spent steam from said open steam-return chamber
of said dry-end steam drying section through said dry-end steam
drying section exit port and said dry-end steam drying section
conveyor return port into said dry-end air/vapor lock chamber;
and
vii. dry-end exhaust control means for regulating the rate at which
said air and said spent steam are exhausted from said dry-end
air/vapor lock chamber by said dry-end exhaust fan; and
f. insulation means for maintaining the temperature level within
said apparatus, wherein said insulation means is affixed between
interior walls of said apparatus and exterior walls of said
apparatus, and wherein said interior walls of said apparatus are
selected to prevent drying steam and spent steam within the
interior of said apparatus from contacting said insulation
means.
18. Process of removing water from cellulosic and textile products,
wherein said water is removed by steam, said process comprising the
steps of:
a. introducing a precharge of water into a steam drying apparatus,
wherein said steam drying apparatus comprises:
i. product-conveying means for receiving and conveying said
products into and through said apparatus, and for transferring said
products away from said apparatus;
ii. a wet-end steam drying section, a plurality of intermediate
steam drying sections, and a dry-end steam drying section, wherein
said wet-end steam drying section, said intermediate steam drying
sections, and said dry-end steam drying section each comprise:
(a) steam-recirculation means;
(b) indirect heating means, wherein said indirect heating means is
positioned in close proximity to said steam-recirculation
means;
(c) an isolated steam-supply chamber, wherein said steam-supply
chamber comprises drying steam application means; and
(d) an open steam-return chamber, wherein said product-conveying
means passes through said open steam-return chamber and wherein
said open steam-return chamber is common to said wet-end steam
drying section, to said intermediate steam drying sections, and to
said dry-end steam drying section;
iii. internal steam-condensing means for condensing an excess of
spent steam, wherein said internal steam-condensing means is
positioned essentially entirely within said open steam-return
chamber;
iv. a wet-end air/vapor lock chamber for preventing the ingress of
air into said wet-end steam drying section, wherein said
product-conveying means first passes through said wet-end air/vapor
lock chamber and into said wet-end drying section;
v. a dry-end air/vapor lock chamber for preventing the ingress of
air into said dry-end steam drying section, wherein said
product-conveying means passes from said dry-end steam drying
section and into said dry-end air/vapor lock chamber; and
vi. insulation means for maintaining the tempurature level within
said apparatus, wherein said insulation means is affixed between
interior walls of said apparatus and exterior walls of said
apparatus, and wherein said interior walls of said apparatus are
sealed to prevent steam within the interior of said apparatus from
contacting said insulation means;
b. heating said precharge of water with said indirect heating means
to form saturated steam;
c. removing air from said apparatus with air removal means until
the atmosphere within said apparatus is comprised substantially
entirely of said saturated steam;
d. superheating said saturated steam to form drying steam by
circulating said saturated steam from said open steam-return
chamber past said indirect heating means utilizing said
steam-recirculation means;
e. measuring the dewpoint within said wet-end air/vapor lock
chamber utilizing a wet-end dewpoint sensing device, and measuring
the dewpoint within said dry-end air/vapor lock chamber utilizing a
dry-end dewpoint sensing device and adjusting the operation of said
steam-recirculation means and said indirect heating means to
regulate the dewpoint within said wet-end air/vapor lock chamber
and the dewpoint within said dry-end air/vapor lock chamber;
f. conveying said products into said apparatus by said
product-conveying means when the dewpoint within said wet-end
air/vapor lock chamber and the dewpoint within said dry-end
air/vapor lock chamber both exceed the dewpoint of the atmosphere
surrounding said apparatus;
g. forcing said drying steam into said isolated steam-supply
chamber utilizing said steam-recirculation means;
h. drying said products conveyed into said apparatus by applying
said drying steam from said steam-supply chamber to said products
as said products pass by on said product-conveying means;
i. withdrawing spent steam away from said products into an upper
portion of said steam-return chamber utilizing said
steam-recirculation means;
j. recirculating a portion of said spent steam past said indirect
heating means to reform said drying steam;
k. redirecting said drying steam, which has been formed by
recirculating said spent steam into said steam-return chamber;
and
l. removing said products from said dry-end steam drying section of
said apparatus ulilizing said product removal means.
19. The process as claimed in claim 18 further comprising the step
of condensing an excess of said spent steam within said open
steam-return chamber, utilizing said internal steam-condensing
means when the dewpoint within said wet-end air/vapor lock chamber
and the dewpoint within said dry-end air/vapor lock chamber both
exceed the dewpoint of the atmosphere surrounding said apparatus by
more than 10%.
20. The process as claimed in claim 19 further comprising the steps
of:
a. monitoring steam condensation rate within said apparatus;
and
b. reducing said steam condensation rate when the dewpoint within
said wet-end air/vapor chamber and the dewpoint within said dry-end
air/vapor lock chamber both exceed the dewpoint of the atmosphere
surrounding said apparatus by less than 5%.
21. The process as claimed in claim 18 wherein said steam-supply
chamber comprises a plurality of steam-supply ducts, wherein said
steam-supply ducts are positioned above and below said
product-conveying means.
22. The process as claimed in claim 21 wherein said products are
molded articles and steam-supply duct faces of said steam-supply
ducts comprise a plurality of steam-supply nozzles, wherein said
drying steam is directed from said steam-supply chamber, through
said steam-supply nozzles and onto said molded articles at a
velocity of about 1000 feet per minute.
23. The process as claimed in claim 21 wherein said products are
thin-weave sheet products and steam-supply duct faces of said
steam-supply ducts are substantially entirely open, wherein said
drying steam is directed from said steam-supply chamber, through
said steam-supply ducts and onto said thin-weave sheet products at
a velocity of about 50 feet per minute.
24. The process as claimed in claim 18 wherein said drying steam is
superheated to a temperature above 375.degree. F.
25. The process as claimed in claim 24 wherein said drying steam is
superheated to a temperature of about 800.degree. F.
26. The process as claimed in claim 18 further comprising the step
of reducing the temperature of said drying steam in said dry-end
steam drying section by reducing the operating temperature of said
indirect heating means in said dry-rnd steam drying section.
27. The process as claimed in claim 26 wherein the operating
temperature of said indirect heating means in said dry-end steam
drying section is about 200.degree. F.
28. The process as claimed in claim 18 further comprising the steps
of:
a. exhausting a mixture of air and an excess of said spent steam
from said wet-end steam drying section through a wet-end exhaust
stack of said wet-end air/vapor lock chamber; and
b. exhausting a mixture of air and an excess of said spent steam
from said dry-end steam drying section through a dry-end exhaust
stack of said dry-end air/vapor lock chamber.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an apparatus and process for drying
cellulosic substances, such as pulp, paper sheet, and molded paper
products, and textiles. More particularly, this invention relates
to an apparatus and method of novel design that uses unsaturated
water vapor (superheated steam) as the means to liberate both
surface and bulk moisture from processed cellulosic and textiles
materials in a continuous process. Even more particularly, this
invention relates to a tunnel dryer designed to use steam as its
drying means and assembling an array of novel features leading to
greatly improved energy efficiency.
2. Description of the Prior Art
As a result of their fabrication process, all cellulosic products,
be they pulp, paper web, or egg cartons, and textile products, have
at some point a very high moisture content needing removal. This
moisture-which typically constitutes 75% of the wet product
weight-includes: (1) moisture on the surface of the product
(surface water); and (2) moisture that is locked into the fibers of
the product (bound water). The traditional means of removing this
water has been to evaporate it by conveying the product through a
tunnel dryer in which a hot gaseous drying medium is directed
against the surfaces of the product, and, for some permeable
materials, through the product. This drying by evaporation is
extremely energy-intensive. The paper and pulp industry alone, with
its large production volume consumes vast amounts of energy each
year just in the drying stage of its production. Consequently, the
reward for improving the drying efficiency is potentially very
great. The applicant believes that the improved efficiency that his
invention permits-particularly in the processing of molded paper
products-represents a significant advance over conventional dryers,
which consume about 120 Therms for every dry ton of product.
Traditionally, the drying medium of choice for tunnel dryers has
been hot, dry air. The hot air technology developed decades ago
during a period when energy costs were low compared to costs of
construction and reflects this fact; it however continues to be
used nearly universally in spite of its increasingly serious
disadvantages. Among these disadvantages is the high cost of
providing the energy to heat air as the drying medium, which,
besides having a relatively low transfer efficiency, must be
continuously replenished-as it is exhausted from the dryer with the
water vapor that it has picked up from the items to be dried. (Once
the air has become laden with water vapor as well as reduced in
temperature, its drying capacity drops precipitously.) In addition,
air as a drying medium has the potential to over-dry the products,
thus embrittling them and/or imposing internal stresses that reduce
their value. What is needed is a drying medium that has a heat
transfer efficiency higher than air and that can be recycled. The
closed system that the recycling requires leads in turn to the
necessity of dealing with the water vapor removed from the products
and entrained in the drying stream-water vapor that traditionally
has just been vented into the atmosphere outside the dryer, with
the energy required to evaporate it. Also needed is a means of
allowing easy tunnel ingress and egress to the conveyor belt while
keeping air out of the tunnel and the hot drying medium in.
The present invention uses gaseous H.sub.2 O at a pressure of one
atmosphere and at temperatures and densities that ensure that the
gas is extremely undersaturated. (Stated differently, the drying
medium is gaseous H.sub.2 O at temperatures far above the dew
point. Such atmospheres-be they at 70.degree. F. or 700.degree.
F.-are referred to as superheated steam.) The invention consists of
the drying process--designed to achieve a very high energy
efficiency--and the apparatus needed to implement the process. How
this is done can best be seen after a more detailed examination of
what is entailed in drying cellulosic (and other fibrous)
products.
When the wet product is introduced into the tunnel, it has water
standing on its surface. The first stage of the drying consists of
the evaporation of that surface moisture. During this first stage,
the rate of water removal remains constant, and at a level that is
a function of the mass per unit time of drying medium impinging on
the product. Surface moisture removal continues at this constant
rate until the surface moisture is gone, by which point the
product's total moisture content is reduced to approximately 30% of
the total weight of the product (the exact percentage at this stage
depending on the particular item involved). The moisture remaining
is in the bulk of the product, contained in its fibers; the removal
of the bulk moisture depends upon capillary action to draw it along
the fibers up to the surface, where it is vaporized by the drying
medium. As a rule, the rate of water removal falls precipitously
and continues to fall as the bulk moisture is being removed. The
amount of energy required to liberate this bulk moisture is a
function of the length of the fibers of the product and other
factors affecting the "wick efficiency." In the dryers currently
used in the pulp and paper industry-those using air as the drying
medium-the energy required to remove the bulk moisture is
approximately equal to that required to remove the surface
moisture, in spite of the latter comprising a much larger quantity.
Unlike the case of the surface moisture, the rate of removal of
bulk moisture is not directly proportional to the rate at which the
drying medium impinges upon the product.
When air is used as the drying medium, the product (consisting
either of discrete items such as molded paper/pulp products or of a
continuous sheet such as paper web, raw pulp or textiles) is
conveyed down the tunnel with a stream of hot, dry air impinging on
it. Because of the once-through path for the air, which comes in
dry and exits carrying away vaporized moisture, the product is
exposed at each step of the way to very low humidity air. Products
thus dried often exit the drying chamber with their moisture
content reduced to approximately 1-3% by weight. Once out of the
chamber and exposed to ambient air, they regain moisture up to some
equilibrium point (about 6-8% moisture by weight), though not
instantaneously and not uniformly. In this way, air as the drying
medium in current systems may over-dry the product to the point
where natural atmospheric conditions replace moisture that the
dryer has had to expend significant energy to remove. If this
over-drying occurs, moisture is reintroduced by the ambient
atmosphere in an uncontrolled fashion that can set up internal
stresses leading to product warpage and other deleterious effects
rendering the product less than satisfactory.
While this problem can be controlled in present hot-air dryers by
regulating the dwell time of the product within the dryer, a
further problem exists with such dryers. In particular, high-speed,
hot-air drying tends to case-harden and warp the surface of the
product. This leads not only to poor product quality, it may also,
in effect, entrap moisture by reducing the wicking efficiency of
the product fibers. As a result, it becomes more difficult for
bound water to escape to the surface to be vaporized. (Entrapping
bound moisture in this manner leads to even poorer quality
products.)
It is well-known that steam (that is, gaseous H.sub.2 O-not to be
confused with the airborne liquid water droplets known vernacularly
by the same name) transfers heat more efficiently than does air.
Steam at a temperature T will exchange more heat with a surface it
is in contact with than will air at the same temperature, all other
things being equal. That is not the whole story, of course, since
it takes more energy to heat up a unit volume of steam to
temperature T than it does to heat up the same volume of air to the
same temperature; also, since the ultimate goal is to dry the
product, the degree of saturation of the steam atmosphere is a very
important parameter to control when steam is the drying medium.
From thermodynamic considerations it is seen that the heat transfer
efficiency depends upon the enthalpy of the fluid (air or steam).
At a pressure of one atmosphere and temperatures above 375.degree.
F., steam has an enthalpy at least 30% higher than that of hot air.
If the gas is a mixture of air and steam, the enthalpy is
intermediate between that for steam and that for air. Consequently,
another consideration that must be dealt with in designing a dryer
using steam as the drying medium is maintaining the purity of the
steam; the extra capital expense involved in such systems can
result in a net loss in efficiency if the steam becomes
significantly contaminated with air. Since it is essential in such
systems that the drying medium be continuously recirculated, the
maintenance of the steam's air-free status is a serious problem; a
small air leak can over a period of time significantly dilute the
recirculating steam. (This is a difficulty that does not arise in
systems where the drying medium just goes once-through and then out
the other end of the dryer.)
As a theoretical idea, the use of steam as a drying medium is not
new. This is true even within the field of pulp and paper drying
(though there is apparently no prior art addressed at drying molded
paper products and the special problems that this entails). See,
for example, Dungler I (U.S. Pat. No. 2,590,849--issued Apr. 1,
1952), which teaches a method for continuous drying using steam as
the medium. Although Dungler I is concerned almost exclusively with
textiles it also alludes to paper and other fibrous materials-but
only those that can be drawn continuously through the drying tunnel
and are sufficiently permeable that the drying medium can impinge
them at high velocity. In particular, Dungler I suggests using
steam to dry thin sheets of material or paper, whereby the item to
be dried is affixed to a permeable conveyor belt and superheated
steam blown through it to liberate bulk moisture. U.S. Pat. No.
2,682,116 issued to Dungler in 1954 (Dungler II) discloses
apparatus for effecting the method disclosed in Dungler I. Both the
apparatus and method claimed in Dungler II relate to very high
impingement velocities and to the establishment, using a complex
vacuum generating system, of a pressure differential across the web
of product to be dried. The apparatus and method of Dungler II
would be completely inapplicable to the drying of molded paper
products, which are not continuous and which are impermeable even
to the high impingement velocities envisioned by Dungler II. In
addition, those high velocities would compromise the position
integrity of such products as they are conveyed through the dryer.
A similar approach is used by Gillis (U.S. Pat. No. 2,760,410),
which discloses particular plumbing and vacuum arrangements for the
use of 400.degree.-1500.degree. F. steam to dry continuous webs of
pervious paper. Luthi (U.S. Pat. No. 4,242,808, 1981), claims a
method and apparatus for using steam to dry paper web, either
pervious or impervious to the drying medium. Luthi recognized the
efficiency associated with superheated steam heating, and goes
further than Dungler I and Dungler II in noting that the steam must
not be contaminated with air if the drying medium is to be as
efficient as possible; nevertheless, Luthi does not set out the
particular techniques that are necessary to ensure minimum air
contamination. Also, Luthi with its high impingement velocities is
inappropriate to the drying of individual items.
Although the process of drying certain pulp and paper products with
steam has been disclosed in principle in the above-cited prior art,
it has rarely if ever been reduced to practice commerically. Within
the molded paper product industry the process has never been
developed even in theory, and it is the molded paper product
industry that will be burgeoning during the coming years and in
need of industrial processes that are far more energy-conserving
than those used in the past. That is, current environmental
concerns associated with plastic containers has led to the
reintroduction by large-scale users of food containers-especially
the fast food outlets-of molded cellulosic articles, bringing pulp
and paper processing plants under increasing pressure to develop
their capacity to handle such products efficiently. More efficient
drying will play a key role in overall efficiency, and it is
submitted that more efficient drying will use steam as a drying
medium. However-and as alluded to above-because molded products are
generally much thicker than paper web and sheet, use of the
prototypical steam dryers taught by the prior art cited above would
require even greater steam velocities or longer dwell times to
drive out trapped moisture. This would increase the likelihood that
the molded articles would be displaced on the conveyor or blown off
completely. A further problem associated with increased steam
output is the greater rate of energy production needed; this in
turn puts a greater premium on steam recovery techniques, something
that has not traditionally been a significant concern. To realize
the goal of greater overall energy efficiency through the use of
steam in connection with the drying process, the energy used to
produce the superheated steam must be recovered from the sheet or
article after the drying has taken place. This leads to still
another problem-the need to provide structurally sound piping
systems and sophisticated steam recovery devices-items that drive
up the cost of a steam dryer and drive down its desirability in any
trade-off analysis. Therefore, while a steam dryer is in theory
more efficient than a hot-air dryer, an evaluation of the two
methods must consider: (1) the possibility that two types of steam
dryers would be required-one for continuous sheet or web and one
for molded articles; and (2) that the hardware required to dry with
superheated steam would be much more expensive.
It is plausible that the failure to implement steam drying even for
pervious and continuous paper webbing is attributable to the costly
dryer sizes and structures required by steam systems. A truly
efficient steam dryer requires a chamber and an associated
air-channelling system, both of which are essentially air-tight and
which are capable of withstanding temperatures above 375.degree.
F., and up to 1600.degree. F. Another problem-and one associated
with pulp and molded articles in particular-involves the increased
difficulty in drying products that are much thicker than simple
fabrics and paper webs. Increased thickness of the item to be dried
means that much higher impingement velocities--or longer dwell
times--are needed to establish and maintain the heat transfer rate
needed to extract the moisture trapped within the product. Higher
velocities tend to blow molded articles off the conveyor that
carries them along in the continuous drying process. Even slight
shifts in the position of the articles are serious since in the
continuous processing of such discrete articles it is extremely
important to maintain position integrity; automatic stackers pick
the articles off the conveyor as they come out of the dryer. These
types of products present still another problem to continuous
drying, either by steam or by air-they vary in size and shape to
such an extent that variations in drying times generally require
varying either the length of the dryer or the dwell-time within the
dryer. Therefore, while the superheated steam drying discussed by
Dungler and Luthi is possible in theory, there are practical
problems associated with the use of steam that must be addressed
before a practical reduction to practice can be achieved.
Furthermore, there are theoretical problems associated with the use
of steam to dry individual molded items that must be addressed.
What is therefore needed is an industrial dryer for paper and pulp
products that is more efficient than conventional dryers,
especially where molded paper products are involved. In particular,
what is needed is an industrial dryer using superheated steam as a
drying medium and that can be effectively integrated into the
complete manufacturing process to the extent that heat and water
generated by the dryer is used in other parts of that process. More
particularly, what is needed is such an industrial dryer which
can--with a minimum of disruption--replace the conventional hot air
dryers currently installed within the textile and pulp and paper
industries, so as to reduce the vast and growing energy expenditure
which that industry demands.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an extremely
efficient superheated steam dryer for all types of pulp and paper
products--molded articles in particular--and textile products as
well. It is a further object of the present invention to provide
continuous drying without the requirement or use of very high
impingement velocities for the drying medium, superheated steam. It
is still another object of the present invention to provide a
superheated steam dryer for the continuous drying of molded
articles in such a way that the individual articles will not be
disturbed by the impinging steam. It is finally an object of the
present invention to provide a superheated steam dryer that can be
utilized for all types of pulp and paper products-despite the size
or shape-in a continuous drying operation and one from which the
low-temperature "waste heat" can be captured for use elsewhere
within the manufacturing process, rather than be exhausted to the
atmosphere as in conventional dryers.
The present invention overcomes the problems associated with the
prior art steam dryers through extensive redesign and optimization
of the dryer hardware. This redesign was premised on: (1) the key
role played by the volume of steam that comes into contact with the
product to be dried; and (2) an analysis of what is involved in
breaking through the product's surface barrier so as to transfer
the heat needed to drive out the moisture trapped with the product
bulk. Driving off a particular quantity of moisture, first from the
surface of a product and then from within the product, requires the
transfer of a particular magnitude of energy which is proportional
to the integrated flux of steam coming into contact with the
surface per unit time at a fixed temperature. Removing the surface
moisture is straightforward; the rate at which the surface moisture
is removed is directly proportional to the total flux of the drying
medium and so to the impingement velocity. Removing the bulk
moisture-once the product surface is dried-is more complicated and
requires dealing with the surface barrier. Ultimately, as the flux
of drying medium (and therefore heat) delivered to the product's
surface increases, the rate of drying becomes limited by the
capillary action within the product. Once that stage is reached,
the drying rate no longer increases in direct proportion to the
velocity of the drying medium but rather with some much smaller
power of the velocity, typically 0.6. Consequently, increasing the
impingement velocity above the level needed to break through the
surface barrier is wasteful.
The present invention uses a steam impingement velocity which is
just enough to break through the surface barrier and deliver the
necessary heat to the bulk of the sheet or article. Whereas the
prior art devices suggest that the velocity of impingement must be
high enough to force the steam into and through the fabric or web
to vaporize the moisture while it is still within the product
(generally at velocities of 7,000-10,000 feet per minute), the
present invention uses just enough velocity to break the barrier
and transfer the necessary heat--at most, that velocity is 3,000
feet per minute. In this way the present invention is a more
efficient steam dryer in that it provides just enough steam to do
the job. The present invention provides, at a minimum, 970 BTU's of
energy in order to evaporate one pound of water from the wet
products. By doing so at efficient impingement velocities, the
present invention reduces the amount of steam that must be
generated at the outset, and it reduces the amount of evaporated
steam that must be recovered.
The present dryer has many features of conventional hot air dryers.
Products to be dried are introduced at one end, conveyed or pulled
through it, and directed out of the dryer at the opposite end. The
drying medium--conventionally air, here steam--is directed onto the
products as they pass through the tunnel. The tunnel comprises
several isolated supply chambers--each with its own set of
impingement nozzles--and a return section which is open the entire
length of the tunnel. Due to the efficient heat transfer obtainable
from steam, these chambers are shorter than are those of
conventional dryers. Also in contrast to conventional dryers, the
system under discussion also contains means within the tunnel for
recovering and reusing the steam originally charged into the tunnel
before the products have been introduced, as well as the steam
generated by the evaporation of the moisture that is located on and
within the products.
Specific questions that had to be addressed in developing a high
efficiency superheated steam dryer applicable to the drying of
individual paper items are as follows: 1) how to recirculate; 2)
how to avoid accidental misalignment of small, lightweight items
positioned on the conveyor belt; 3) how to keep air out of the
recirculating steam; 4) how to preserve the dryer's thermal
insulation; and 5) how to recover and reuse steam originally
charged to the dryer, as well as the steam generated from the
evaporation of the product moisture.
An important feature of the present invention is the means by which
drying steam is applied to wet products at a velocity high enough
to break the product surface vapor barrier, and by which the
"spent" steam is drawn away from those products. The movement and
recirculation of steam is achieved in the present invention by
placing large capacity recirculation fans in the open return
section of each drying section of the tunnel, wherein each drying
section comprises one of the isolated steam supply chambers. These
recirculating fans operate by drawing spent steam away from
products being conveyed through the tunnel. The spent steam, which
is a combination of drying steam originally contained within the
tunnel and the steam resulting from the liberated moisture on or in
the wet product, is drawn into the fan inlet and directed past one
or more indirect steam heaters. In a single-conveyor dryer, the
recirculating fan is located in the ceiling and to one side of the
drying section. There is one indirect heater paired with each
recirculating fan and that heater is located to one side of the fan
in an upper section of the supply chamber. In a double-conveyor
dryer, the recirculating fan is also located in the ceiling of the
drying section, but it is centered in the tunnel. In this way, two
indirect heaters, each located on either side of the fan, heat the
spent steam which the fan draws from the return section. The
recirculating fan then forces the heated steam into the supply
chamber for application to the next wet products passing through.
In order to accommodate the variety of types of products, the
recirculating fan is of the type that can be operated by variable
control means. This variability enables the operator to vary the
product dwell time within the dryer as a function of the product
and the need to maintain product position integrity on the
conveying means.
It is essential that small articles participating in a continuous
production process maintain their exact positions on the dryer
conveyor while passing through the tunnel. Efforts to meet this
concern with current hot-air dryers have presented many problems.
One approach has been to position the supply orifices above and
below the belt on which the article travels, and to align them
pairwise in an attempt to balance the forces exerted on the molded
paper products-which are progressively lighter as they are divested
of their moisture. This localization of the hot air steam can
result in cold spots, and irregular drying. The present invention
approaches the turbulence problem so that the orifices can be
positioned in a way that avoids the cold-spot problem
completely.
The key step making the turbulence reduction possible without the
introduction of uneven drying was to make the supply orifices much
larger than they have traditionally been. (Indeed, they are perhaps
better referred to as supply ducts, such is their size and shape.)
In this way a much lower impingement velocity is possible even
while maintaining the volume of steam delivered to the product per
unit time--and per unit area--at a high level. The limiting
constraint is only that the velocity be high enough to break the
"surface barrier" and thus permit the drying medium to get
sufficiently close to the item so that efficient heat transfer is
ensured along with a low humidity atmosphere directly next to the
item's surface. In designing the flow rate for the drying medium it
was observed that impingement velocities higher than this are
wasteful. Besides causing unnecessary turbulence, these higher
impingement velocities deliver heat to the system that is not
efficiently used. It is found that an impingement speed of
approximately 1000 feet per minute is necessary and sufficient to
break the surface vapor barrier. To the extent that the resulting
flux of drying medium is inadequate to dry the items, one simply
increases the dwell time of the items within the tunnel. This is
far more efficient than increasing the impingement velocity;
furthermore, with the efficiency increase delivered by the present
invention, dwell times are decreased from those necessary with hot
air dryers, even with their higher impingement velocities. One goal
of the present invention is to provide only enough steam to dry the
product and at a velocity that is just enough to break through the
surface vapor barrier of the product and deliver the necessary heat
to the bulk moisture. Through optimization of the steam supply
hardware the quantity of steam delivered to the product is
therefore minimized and the present superheated steam drying system
is at its most efficient. Simultaneously, the cold-spot problem is
fixed by positioning the orifices so that the impinging steam hits
the article at a uniform temperature. In part this optimization is
achieved by accurately sizing the orifices, and in part by
positioning them at specific variable distances-distances that
depend upon the nozzle to conveying means distance and the type of
product to be dried-and by offsetting them so that as the conveyor
belt moves through the tunnel different portions of it are
uniformly under an orifice. As a result, a uniform volume of steam
is imparted to a uniform section of the conveyor in a manner that
provides a constant low-velocity impingement to the product being
conveyed through the dryer.
Another novel aspect of the hardware of the present invention
involves the steam return system. Conventional dryers have exhaust
systems for the returning gases. Because the hot air (or steam)
liberates a liquid from the product by vaporizing surface moisture
and entrained moisture, the volume of gas (a mixture of air and
water vapor) exiting the drying region is greater than the volume
of gas impinging the product. If the exit area is equal to, or less
than, the supply area, the increased volume of gas must leave at a
higher velocity than the entering gas, resulting in turbulence
around the product. For sheet products that are affixed to the
conveyor this is not a problem, since turbulence will not affect
their position integrity. For molded articles, however, this
turbulence knocks the articles out of position and makes it
impossible to use automated stacking after the drying process.
Besides going to large orifices and, consequently, low impingement
velocities, the present invention attacks the turbulence problem by
redesigning the means by which spent steam may exit the tunnel. In
the present invention, the regions immediately adjacent to the
supply ducts in both the top and bottom of the tunnel are open and
comprise "ducts" by which the spent steam and evaporated vapor can
exit the tunnel. The cross sectional area of the exhaust ducts is
so much larger than that of the supply orifices that the exiting
gas can move at a low velocity and therefore minimize turbulence.
In this manner, instead of being deflected and contributing to the
item-moving turbulence, the spent steam and entrained evaporation
vapor simply exit the tunnel and are conducted to a recycling
station. This is an important factor in allowing the impingement
velocity to be as high as it is without shifting the light
items.
An added benefit of the wide-open exit area is that there is a
significant reduction in the piping required in the manufacture of
the superheated steam dryer of the present invention. Besides
enabling the present invention to dry molded articles with
superheated steam, these unique features result in a dryer which
uses steam much more efficiently than the prior art steam dryers
previously discussed. Further, the hardware disclosed in the
present invention is more economical to manufacture than prior art
devices because there is no need for extensive return piping and
many exhaust fans, all of which must have the structural integrity
to withstand the severe environment of superheated steam. In this
way, the present invention provides a steam dryer that is
commercially feasible and that can be used for all cellulosic
substances-from pulp and molded articles to paper web and
fabrics.
Much of the advantage of the hot steam as a drying medium is lost
if there is any significant contamination by air. Since the steam
is continually being recirculated with every gas that becomes
entrained within it, even small air leaks are serious. There are
two primary sources of contaminating air: the shafts of the many
recirculating fans and the ingress/egress ports for the belt(s).
The standard high capacity industrial fans suited for commercial
paper/pulp dryers tend to have a leak rate along the shaft of about
25 cubic feet per minute (cfm), given the pressure difference
between the ambient atmosphere and the pressure within the dryer
near the shaft. The present invention required much lower leakage
rates than this; thus it incorporates high volume fans designed for
it to have a leakage rate of about one cfm under the conditions
specified above.
The more serious problem--and one entailing a less mundane
solution--was presented by the ingress/egress ports of the dryer.
There have been some prior art attempts to deal with this problem
by either using tight-fitting slits (nips) for the entrance and
exit of the product and the natural overpressure existing because
of the evaporated vapor. Obviously the nips, while possible for
continuous paper web, are impossible when the product entering and
leaving the tunnel is comprised of individual items resting atop a
conveyor belt. Also, the air-exclusion-through-overpressure
solution is not tenable in a system designed to optimize energy
efficiency. It is a major goal of the applicant to recapture, to
the greatest extent possible, the heat introduced with the steam;
this includes that fraction of the steam heat was is transformed
into latent heat of vaporization contained in the evaporated
moisture.
The present invention introduces novel "air/vapor locks" at both
the ingress and egress ports of the tunnel. The conveyor belt
passes directly through these air/vapor locks, which are in direct
communication with the ambient room air, on one side, and the
tunnel air on the other. More specifically, the air/vapor lock at
the ingress is a transition stage between the room which contains
the tunnel and the first tunnel segment. It is open to mass
transfer from both sides. Air comes in from the room and air-free
steam comes in from the tunnel segment. (An identical arrangement
exists at the far end of the tunnel, with that air/vapor lock
serving as a transition between the final tunnel stage and the
room.) The mixture of gases within the air/vapor locks is exhausted
through the tops thereof, thus setting up a strong vertical flow
within the air/vapor lock interior. This vertical circulation
prevents air from passing (horizontally) from the room into the
tunnel. Although the air/vapor lock system depends for its
operation on a slight overpressure within the tunnel, there is a
feedback mechanism used that leads to the minimization of steam
outflow from the tunnel. The feedback system depends upon a
dewpoint sensor placed in an exhaust means of the air/vapor lock,
where the air stream and the steam stream are well mixed. If the
volumes of gas contributing to the mixture are equal, the total
absolute humidity (H.sub.2 O mass per unit volume) sensed should be
half-way between the absolute humidity of the room and the absolute
humidity of the tunnel. If that measured humidity departs from what
has been determined to be optimum (i.e., minimum outflow from
tunnel combined with zero "leakage" through the air/vapor lock to
tunnel), the exhaust fan speed can be varied or the condensation
rate within the tunnel (see below) varied.
Most of the energy that recirculating steam introduces into the
items to be dried is converted into latent heat of vaporization as
moisture evaporates from those items. To recover that energy, it is
necessary to condense the resultant vapor; for that purpose a steam
condenser is placed in the steam return section of the tunnel. The
heat of vaporization is dumped into the cooling water which then
conveys it to other stages of the production process via heat
exchanger means. In addition, the hot condensate--at temperatures
typically about 200.degree. F.--may be conveyed from the floor of
the tunnel, through a cleaning procedure and then over to those
stages of the manufacturing process that require hot water. In this
way, most of the water needed in the production process is
recycled, rather than being discharged into the atmosphere. This is
a conservation measure that is fairly easy to implement in a dryer
using steam as the drying medium, but which is all but impossible
to implement in air dryers, where the absolute humidity even in the
presence of the evaporated moisture is much lower.
Besides increased efficiency in energy recovery which the internal
condenser makes possible (that prior art that used
energy-recovering condensers at all located them outside of the
dryer), the cooling that it introduces is beneficial since this
ensures that the dried products exit the tunnel at a temperature
more appropriate for handling and for exposure to the room air
(which can support combustion, should the paper product be at its
ignition temperature). The condensation rate--which is
adjustable--is established using humidity measurement means in the
air/vapor locks as well as humidity measurement means in the room
containing the dryer. When the humidity within the air/vapor locks
exceeds the humidity in the room, the rate of internal condensation
may be increased.
Although there is not a significant vapor pressure difference
between the inside and outside of the dryer, one that would support
a convective flow of steam out the inside wall, there is a very
great difference in atmospheric water content--that is, in water
vapor pressure. Efforts must therefore be made to prevent the
inside air from diffusing or otherwise migrating through the walls.
The walls contain six inches or more of high-temperature thermal
insulation, which quickly loses its insulating qualities once it
becomes wet, as it would as the steam passed into it and then
condensed. Because of these considerations, all the sections of the
inner tunnel wall must be welded together with a continuous
bead.
The present invention thus provides an extremely energy-efficient,
continuously-operated steam dryer for all types of processed
cellulosic and textile products. The novelty and utility of the
present invention resides in its ability to retain steam within the
dryer and maximize the heat transfer efficiency of that steam by
minimizing the amount of contaminant air permitted to enter the
dryer. Another novel and useful feature of the present
invention--one that sets it apart from prior art dryers--is its
ability to recover the latent energy of spent recirculating steam,
as well as the steam formed by the liberation of moisture from the
product being dried, while that steam is contained within the
dryer. These novel features and others will become apparent upon
review of the detailed description of the preferred embodiment.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view of a steam dryer of the present
invention, with a double-conveyor system.
FIG. 2 is a cross sectional side view of a steam dryer of the
present invention, with a single-conveyor system.
FIG. 3 is a top view of a steam dryer of the present invention,
with a single-conveyor system.
FIG. 4 is a cross-sectional view along the width of a drying
section of a steam dryer of the present invention, with a
single-conveyor system.
FIG. 5 is a cross-sectional view along the width of a drying
section of a steam dryer of the present invention, illustrating the
nozzle arrangement within the drying section for a single-conveyor
system.
FIG. 6 is a side view of the supply duct arrangement within a steam
supply chamber of the present invention, illustrating top and
bottom supply ducts for a single-conveyor system.
FIG. 7 is a cross-sectional view along the width of a drying
section of the present invention, with a double-conveyor
system.
FIG. 8 is a perspective view of the supply duct arrangement for one
section of a double-conveyor system.
FIG. 9 is a top view of the nozzle arrangement for two adjacent
supply ducts.
FIG. 10 is a cross-sectional view of the fan shaft seal of the
present invention.
FIG. 11 is a cross-sectional view of the internal steam-condensing
means of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
A steam dryer 1 of the present invention, as illustrated in FIG. 1,
operates continuously to dry a multiplicity of types of pulp and
paper products and textile products (generally identified as
products 100), wherein said products 100 are either pulled through
said steam dryer 1, or conveyed by one or more conveyors 2.
Generally, and as illustrated in FIGS. 1 and 2, said steam dryer 1
comprises a wet-end air/vapor lock chamber 9, a dry-end air/vapor
lock chamber 10, a plurality of steam drying sections 22, internal
steam-condensing means 5, maintenance access doors 77, and a
cooldown exhaust stack 75. Each one of said steam drying sections
22 contains indirect steam heating means 19, steam-recirculation
means 13, a plurality of compartmentalized steam-supply chambers 3,
and sections of a single, open steam-return chamber 4. In the
preferred embodiment, said indirect steam heating means 19 is a
gas-fired indirect heater 19, and said steam-recirculation means 13
is a variable-speed, centrifugal plug fan 13.
The dryer 1 illustrated in the FIGURES is the type that has said
conveyors 2 as product conveying means for moving products into and
out of said steam dryer 1. It is to be understood however, that
pull-through and float-through types of said dryer 1 comprise
essentially the same components as described herein. In particular,
said wet-end air/vapor lock chamber 9 comprises either one or two
wet-end product entry ports 6 and a corresponding number of wet-end
conveyor belt return ports 7, all of which are centered in a
rectangular section 14. A wet-end dewpoint controller device 8 is
attachable to an outside wall 11 of said wet-end air/vapor lock
chamber 9, and a wet-end dewpoint sensing device 78, which provides
a measure of the dewpoint within said wet-end air/vapor lock
chamber 9, is positioned within said wet-end air/vapor lock chamber
9. (Said wet-end controller device 8 compares the dewpoint within
said wet-end air/vapor lock chamber 9 with the dewpoint of the
atmosphere surrounding said steam dryer 1. Based upon the
information obtained, said wet-end dewpoint controller device 8 and
a dry-end dewpoint controller device 69 control the operation of
said internal steam-condensing means 5.) In the preferred
embodiment, said rectangular section 14 is approximately 3 feet
long and its width and height are dependent upon the drying load
and whether one or two conveyors 2 have been incorporated into the
design of said steam dryer 1; if there is only one conveyor, the
width of said rectangular section 14 is about 6 feet and the height
is about 9 feet; if there are two conveyors, the width is about 9
feet and the height is about 12 feet. A first tapered section 15 is
affixed to a top 79 of said rectangular section 14, and is
connected to a first transitional section 16. Said transitional
section 16 is then connected to a wet-end exhaust stack 17, which,
in turn, is affixed to a wet-end exhaust fan 18. Said exhaust fan
18 draws a gaseous mixture from said wet-end air/vapor lock chamber
9, where said gaseous mixture comprises air from the atmosphere
surrounding said steam dryer 1 and spent steam emitted from said
steam-return chamber 4 of a wet-end steam drying section 53. Within
said wet-end air/vapor lock chamber 9, said rectangular section 14
contains wet-end steam drying section entry port 20 and wet-end
steam drying section conveyor return ports 21, which are located at
a side opposite from said wet-end product entry port 6 and said
wet-end conveyor belt return port 7. Said products 100 enter said
wet-end steam drying section 53 via said entry ports 20, and said
spent steam exhausts from said steam-return chamber 4 into said
wet-end air/vapor lock chamber 9 via said entry ports 20 as well as
said return ports 21. Any steam condensing within said wet-end
air/vapor lock chamber 9 is removed from said chamber 9 via a
wet-end air/vapor lock drain 90.
As previously noted, said steam dryer 1 may be used to dry any of a
number of types of pulp, paper and textile products. In order to
accommodate the variation in drying requirements, the present
invention may be formed by connecting a plurality of steam drying
sections 22 configured in the same manner as said wet-end steam
drying section 53. The length of said dryer 1 is generally
determined by the desired product capacity as well as the moisture
content of the product and the required dwell time the product must
remain within said dryer 1 to reduce the moisture content of the
product. Typically, for molded articles, the length of said dryer 1
is about 126 feet; for thinner products and products for which
there is no concern of article displacement, the length of said
dryer 1 may be less. When said steam dryer 1 operates with a single
conveyor 2, said steam drying sections 22 are designed as
illustrated in FIGS. 3-5. In the preferred embodiment of such a
single conveyor dryer 37, said steam drying sections 22 are about
12 feet long.times.6 feet 6 inches wide.times.8 feet in height, and
rest on 6-inch steel channels 23. In a double conveyor dryer
38--generally illustrated in FIGS. 1, 7 and 8--the length remains
the same; however, the width is 12 feet and the height is 15 feet.
Left-side interior walls 24 and right-side interior walls 25 of
said drying section 22 are fabricated of aluminized steel. In order
to maintain the operating temperature within said steam dryer 1 at
a desired level, 6 inches of insulation means 101 is contained
between all interior walls 24 and 25, and exterior walls 26 of said
steam dryer 1, and all of said interior walls are welded together
to seal the interior of said steam dryer 1 so that the integrity of
said insulation means 101 will be maintained. Within said steam
drying sections 22, said recirculating plug fan 13 draws return
steam from said steam-return chamber 4 and forces it past said
heating device 19. The heated steam is then directed into three
steam-supply chambers 3, each of which is the same width and height
as any of said drying sections 22, but only one-third of the length
of said drying sections 22.
As illustrated in FIG. 3, in said single conveyor dryer 37, said
plug fan 13 is located in a center one-third portion of the length
of said drying section 22, and is positioned about 8 inches from
said left-side interior wall 24. On the other hand, and as
illustrated in FIGS. 1 and 7, in said double conveyor dryer 38,
said plug fan 13 is still located in said center one-third portion
of the length of said drying section 22, but it is in a centered
one-third portion of the width of said drying section 22 as well.
Thus, for said double conveyor dryer 38, said plug fan 13 will
supply steam to supply steam chambers 3 that are located along both
of said interior walls 24 and 25 of said drying section 22.
An essential feature of said plug fan 13 is a plug fan shaft seal
27 utilized to reduce to about 1-3 cubic feet per minute (CFM) the
air in-leakage from the essentially dry-air atmosphere surrounding
said steam dryer 1 into said steam drying section 22. In the
preferred embodiment, said shaft seal 27, which seals a plug fan
shaft 76, comprises a series of self-lubricating graphite-based
rings 80 positioned adjacent to a compression spring 81, as
illustrated in FIG. 10. This design permits said shaft seal 27 to
seal said shaft 76 tightly over extended periods of time. Each of
said plug fans 13 located within each of said drying sections 22
operates to provide a supply of steam to said steam-supply chambers
3. In said single conveyor dryer 37 supply steam is forced into a
right-side steam-supply shaft 30 within said drying section 22,
wherein said right-side steam shaft 30 runs along said right-side
interior wall 25, is about 1 foot 4 inches in width, and is common
to open supply ends 31 of steam-supply ducts 28 and 29 of each of
said steam-supply chambers 3. In said double conveyor dryer 38, in
addition to said right-side supply shaft 30, there is a left-side
steam-supply shaft 39, located along said left-side interior wall
24. Said left-side supply shaft 39 provides an additional volume of
drying steam to said steam-supply ducts 28 and 29 via double
conveyor open supply ends 36.
Within each of said steam-supply chambers 3 is a plurality of top
steam-supply ducts 28, and a corresponding number of bottom supply
ducts 29, all of which are fabricated of aluminized steel. A key
feature of said steam dryer 1 of the present invention is that
steam is supplied to said products 100 utilizing a plurality of
individual supply ducts 28 and 29, which are spaced 4 inches apart,
rather than the single supply ducts used in the prior art. Said
individual supply ducts 28 and 29 permit large volumes of spent
steam to be drawn away from said products 100 and sent to said
return chamber 4 by said plug fan 13 from between said supply ducts
28 and 29 via steam-return path 33, as well as by conventional
routes such as return paths 34 and 35 of said drying section
22--illustrated in FIGS. 4-6 and 8, thereby reducing steam
turbulence about said products 100.
Said supply ducts 28 and 29, illustrated in FIGS. 5 and 6, are
about 8 inches wide and in said single conveyor steam dryer 37 said
supply ducts 28 and 29 are about 4 feet 4 inches in length. In said
single conveyor steam dryer 37, said top supply ducts 28 taper from
a rectangular cross-section of about 1 foot.times.8 inches at said
open supply end 31, to a rectangular cross-section of about 6
inches.times.8 inches at a closed supply end 32. This taper is
necessary in the single conveyor dryer 37 because steam is supplied
to said supply ducts 28 and 29 from only one direction--that is,
from said right side steam-supply shaft 30. As steam is transferred
to said products 100 on said conveyor 2, the volume of steam in
said supply ducts 28 and 29 is reduced and a pressure drop occurs.
In order to maintain the desired steam velocity impinging on said
products, this pressure drop must be minimized. Tapering of said
supply ducts 28 and 29 results in a minimization of the pressure
drop because it reduces the supply duct volume. In this way, the
steam impingement velocity is maintained at the desired level along
the entire 4-foot 4-inch length of said supply ducts 28 and 29. In
said double conveyor dryer 38 there is no pressure drop problem to
overcome, because steam is supplied to said supply ducts 28 and 29
from two directions--that is, from said left-side supply shaft 39
and said right-side supply shaft 30, as illustrated in FIG. 7. The
drying sections 22 of said double conveyor dryer 38 are about 8
inches wide and about 9 feet long. Also, said supply ducts 28 and
29 of said double conveyor dryer 38 have constant cross-sectional
dimensions 1 foot 4 inches. These ducts are not tapered because
sufficient volumes of steam are supplied to the entire length of
said double sets of supply ducts 28 and 29 to maintain the desired
steam impingement velocity.
In order to dry pulp and molded articles in particular, said steam
dryer 1 of the present invention directs superheated steam from
said supply ducts 28 and 29 directly onto said products 100 via
steam-supply nozzles 40. On said top supply ducts 28, said supply
nozzles 40 are located at a top supply duct face 41, and are
positioned about 6 inches above said conveyor 2. On said bottom
supply ducts 29, said supply nozzles 40 are located at a bottom
supply duct face 42, and are positioned about 2 inches below said
conveyor 2. In the preferred embodiment, said supply nozzles 40 are
2-inch high tubes, with inside dimensions of about 2 inches.times.2
inches. Said nozzles 40 preferably are made of aluminized steel,
and are welded into supply orifices 93 in said top supply duct face
41 and said bottom supply duct face 42, such that they extend about
1 inch beyond said duct faces 41 and 42. Nozzles 40 of said top
duct face 41 and nozzles 40 of said bottom duct face 42 are in
direct alignment with each other, corresponding to aligned top
supply ducts 28 and bottom supply ducts 29. It is to be understood
that said nozzles 40 are required to produce adequate steam
impingement velocities to reach the moisture bound within the
fibers of pulp and molded articles--which tend to be the thickest
products. Such high velocities are not required to dry thinner
textile and paper weaves. In such products, drying may be achieved
with the same steam flux at a much lower velocity by forming said
top supply duct 28 and said bottom supply duct 29 with supply duct
faces 41 and 42 that are substantially entirely open.
As illustrated in FIGS. 5 and 9, said nozzles 40 of the present
invention are arranged to optimize steam utilization by directing
just enough steam onto the surface of said conveyor 2 to uniformly
impinge said products 100 being conveyed. This is achieved by
offsetting the positions of said nozzles 40 for each pair of two
sets of top and bottom supply ducts 28 and 29. Specifically, and as
depicted by the nozzle layout illustrated in FIG. 9, a left-side
edge 82 of a first left-side exemplar nozzle 43 of a first exemplar
supply duct 44 is positioned 11/2 inches from a left-side edge 83
of said first exemplar supply duct 44 and said first left-side
exemplar nozzle 43 is centered 11/4 inches from a leading edge 45
of said first exemplar supply duct 44. The remainder of said
nozzles 40 positioned along said leading edge 45 of said first
exemplar supply duct 44 are equally spaced 6 inches apart. A
left-side edge 84 of a second exemplar left-side nozzle 46 of said
first exemplar duct 44 is positioned 51/2 inches from said
left-side edge 83 of said first exemplar supply duct 44 and said
second exemplar left-side nozzle 46 is centered 11/4 inches from a
trailing edge 47 of said first exemplar supply duct 44. The
remainder of said nozzles 40 positioned along said trailing edge 47
are equally spaced 6 inches apart. A second exemplar supply duct
48, paired with said first exemplar supply duct 44, comprises
nozzles 40 of the same general arrangement as those of said first
exemplar supply duct 44. A left-side edge 85 of a third exemplar
left-side nozzle 49 on a leading edge 50 of said second exemplar
supply duct 48, is positioned 31/2 inches from a left-side edge 86
of said second exemplar supply duct 48 and a fourth exemplar
left-side nozzle 51 on a trailing edge 52 of said second exemplar
supply duct 48, is positioned 71/2 inches from said left-side edge
86 of said second exemplar supply duct 48. With this nozzle
arrangement, steam impinges said products 100 uniformly with
velocities of 1000 FPM or higher.
As previously stated, said drying steam, the volume of which is
increased by steam produced by liberating moisture from said
products 100, is then pulled by said plug fans 13 into said
steam-return chamber 4 via return paths 33-35. The steam drying
method described herein requires that a constant drying steam flux
be used to dry said products 100. Since there is an overabundance
of steam produced in the drying process, excess steam must be
provided an escape path from within said individual drying sections
22. This is achieved in the present invention by leaving said
steam-return chamber 4 open along the entire length of said steam
dryer 1, regardless of the number of adjoining drying sections 22.
While some of the excess steam exits said wet-end steam drying
section 53, most of it diffuses from leading drying sections to
latter drying sections, i.e., from the highly steam-concentrated
sections to the less steam-concentrated sections, via said return
chamber 4. Some of this excess steam is recirculated back to said
steam-supply chamber 3 and the rest comes in contact with said
internal steam-condensing means 5. Excess steam contacting said
internal steam-condensing means 5 experiences a resultant reduction
in both temperature and volume, thereby maintaining the internal
pressure of said steam dryer 1 at slightly more than one
atmosphere. At the same time, the energy of the excess steam, which
is released when the excess steam temperature is reduced, is then
recovered by cooling water of said internal steam-condensing means
5.
As illustrated in FIG. 11, in the preferred embodiment of the
present invention said internal steam-condensing means 5 extends
from said wet-end steam drying section 53 to a dry-end steam drying
section 54. Said steam condensing means 5 comprises a cooling water
pump 55, a cooling water tube 62, a condensate tray 56, and a
condensation control valve 60. Said cooling water pump 55 will vary
in size as a function of the quantity of cooling water moved
through said cooling water tube 62, and the size of said cooling
water tube 62 is dependent upon the quantity of steam within said
steam dryer 1 that must be condensed. (Although illustrated as a
single tube, said cooling water tube 62 may comprise a plurality of
tubes extending along a floor 59 of said return chamber 4.) Said
cooling water tube 62 enters said return chamber 4 via a wet-end
cooling tube orifice 57 located in said wet-end drying section 53,
and said cooling water tube 62 exists said return chamber 4 via a
dry-end cooling tube orifice 58 located in said dry-end steam
drying section 54. Said condensation control valve 60, affixed to
said cooling water tube at a dry-end control position 61, is used
to regulate the flow of cold water through said cooling water tube
62, thereby controlling the rate of excess steam condensation. Said
control valve 60 is connected, by well-known means, to said wet-end
dewpoint controller device 8 and said dry-end dewpoint controller
device 69, wherein the opening and closing of said control valve 60
is regulated by Proportional-Integral-Derivative (PID) means. The
cooling water--which exits said dry-end steam drying section 54 via
said cooling water tube 62 through said dry-end cooling tube
orifice 58--is transferable to other pulp and paper processing
locations via a main outlet line 63. Said cooling water tube 62 is
made of aluminized steel and is welded to said orifices 57 and
58.
Said condensate tray 56 is about 2 feet wide, extends from said
wet-end cooling tube orifice 57 to said dry-end cooling tube
orifice 58, and is fabricated of aluminized steel. Said condensate
tray 56 is located directly under said cooling water tube 62, where
it is utilized to gather hot water condensate dripping from said
tube 62. Said hot water condensate exits said tray 56 via a
drainage port 64, and is transferable either to a main sewer line,
or to a heat recovery device, via a condensate return line 65. Said
condensate return line 65 is about 3 inches in diameter and exits
said steam dryer 1 through a condensation line orifice 66 located
at about the midpoint of said floor 59 of said steam-return chamber
4. The quantity of excess steam reduced to condensate and removed
via said condensate return line 65 is regulated by said wet-end
dewpoint controller device 8 and said dry-end dewpoint controller
device 69, both of which operate to control the amount of cooling
water running through said cooling water tube 62 via said control
valve 60.
The operation of said internal steam-condensing means 5 is
regulated to condense most, but not all, of the excess steam in
said steam dryer 1. A small percentage of the excess steam located
within said return chamber 4 of said dry-end steam drying section
54 is vented from said dry-end steam drying section 54 into said
dry-end air/vapor lock chamber 10, via dry-end steam drying section
exit ports 67 and dry-end steam drying conveyor return ports 68.
Said dry-end air/vapor lock chamber 10 is configured in essentially
the same manner as said wet-end air/vapor lock chamber 9, wherein
the function of said dry-end air/vapor lock chamber 10 is to
prevent dry air in-flow into said dry-end steam drying section 54
of said steam dryer 1. A dry-end dewpoint sensing element 79,
attachable to an inside wall 70 of said dry-end air/vapor lock
chamber 10, measures the dewpoint of the atmosphere within said
dry-end air/vapor lock chamber 10 and relays that information to
said dry-end dewpoint controller device 69. Said dry-end dewpoint
controller device 69 compares the dewpoint within said dry-end
air/vapor lock chamber 10 with the dewpoint of the atmosphere
surrounding said steam dryer 1. As previously stated, this
information is utilized to send a command signal to said control
valve 60. A gaseous mixture of excess steam emitted from said
dry-end steam drying section 54 and air coming into said dry-end
air/vapor lock chamber 10 via dry-end product exit ports 71 and
dry-end conveyor return ports 72 from the atmosphere surrounding
said steam dryer 1 is pulled from said dry-end air/vapor lock
chamber 10 by a dry-end exhaust fan 73 and exhausted through a
dry-end exhaust stack 74. Any steam condensing within said dry-end
air/vapor lock chamber 10 is removed via a dry-end air/vapor lock
drain 91.
The superheated steam drying process of the present invention
comprises the incorporation of said steam dryer 1 into the process
of removing moisture from said products 100. Whether the particular
product is in sheet form, such that it can be restrained for
steam-through drying, or floated through said dryer 1, or whether
the product must be conveyed through said dryer 1 on said conveyor
2, such as with pulp and molded articles, the drying process of the
present invention can provide the appropriate amount of energy--in
an efficient manner--to dry such products quickly, and the present
invention can make those products better qualitatively. As
previously stated, this is done by drying with a drying medium that
is essentially entirely air-free, and consisting almost completely
of very hot, unsaturated gaseous water--i.e., superheated
steam.
Prior to introducing pulp and paper products to this superheated
steam environment, air must be eliminated from said steam dryer 1
and steam must replace the eliminated air. This is done by
introducing a precharge of water directly into each of said drying
sections 22 of said dryer 1. The size of the water charge in said
drying sections 22 is a function of the number of said drying
sections 22, but at least large enough so that its volume when
converted to steam is sufficiently great that it fills all of said
steam dryer 1. At the same time that said precharge of water is
introduced to said steam dryer 1, said indirect heaters 19 are
turned to their maximum operating temperatures of about
2000.degree. F., and said plug fans 13 are turned on. Furthermore,
said wet-end air/vapor lock exhaust fan 18 and said dry-end
vapor-lock exhaust fan 73 are turned on and used to draw the
"contaminant" air from said drying sections 22. As said heaters 19
vaporize the water within said dryer 1, and said plug fans 13
continuously recirculate the vaporized water past said heaters 19,
said steam becomes superheated and its temperature exceeds
800.degree. F. As said steam is being superheated, said wet-end
dewpoint sensing element 78 measures the dewpoint within said
wet-end air/vapor lock chamber 9 and said dry-end dewpoint sensing
element 79 measures the dewpoint within said dry-end air/vapor lock
chamber 10. In turn, said wet-end dewpoint controller device 8 and
said dry-end dewpoint controller device 69 compare those dewpoints
with the dewpoint of the atmosphere surrounding said dryer 1. When
the dewpoint within each of said air/vapor lock chambers 9 and 10
exceeds the surrounding dewpoint by more than 10%, and the
temperature of the drying steam within said drying sections 22 is
about 800.degree. F., said wet products 100 are introduced to said
steam dryer 1 via said product entry port 6.
As moisture-laden products 100 are conveyed or pulled into said
wet-end drying section 53, they are at a temperature of about
90.degree. F. When introduced to the drying steam, which is at a
temperature in excess of 800.degree. F., the product temperature
ramps up to, or near, the temperature of the steam. The rate of
product temperature increase is a function of the features of the
product itself. As previously stated, said dryer 1 may be formed of
a multiplicity of said drying sections 22. As said products 100 are
conveyed through said drying sections 22, the operating rate of
said plug fans 13 is controlled to introduce a sufficient volume of
steam into said steam-supply chambers 3 to dry the particular
products being conveyed. For molded articles the recirculation rate
is about 120,000 CFM near the wet-end of said dryer 1 and about
80,000 CFM near the dry-end, where the articles are lighter in
weight. For paper weaves, the recirculation rate near the wet-end
is about 80,000 CFM, and near the dry-end it is about 40,000
CFM.
As previously noted, nozzles 40 of said top and bottom supply ducts
28 and 29 are required to increase the velocity of the drying steam
used to impinge the surface of thick molded articles and pulp. In
the preferred process of the present invention, within said wet-end
drying section 53, the steam exiting said nozzles 40 of said top
supply ducts 28 impinges said products 100 at a speed of about 1200
FPM. From said nozzles 40 of said bottom supply ducts 29, the steam
impingement velocity is only about 500 FPM, primarily because the
steam from said bottom ducts 29 is diffused by said conveyor 2.
However, within said dry-end steam drying section 54, the steam
impingement velocities are reduced to about 800 FPM and 300 FPM,
respectively, to compensate for the reduction in weight of said
products 100 at that end of said steam dryer 1. For much thinner
products, wherein much lower velocities can achieve the same
drying, and higher velocities are undesirable, said supply duct
faces 41 and 42 are substantially entirely open and the steam
impingement velocity is about 50 FPM throughout said steam dryer
1.
Said plug fans 13 also draw a mix of the steam of the moisture
liberated from said products 100 and the original drying steam into
said steam-return chamber 4 via return paths 33-35. This spent
steam is recirculated past said indirect heaters 19 and into said
supply chambers 3, to be used as drying steam again. The remaining
excess steam diffuses through said return chamber 4, which is
common to all of said drying sections 22, toward said dry-end steam
drying section 54. As the steam drying process of the present
invention continues, and said products 100 are conveyed through
said dryer 1, the dewpoint within said wet-end air/vapor lock
chamber 9 and the dewpoint within said dry-end air/vapor lock
chamber 10 are continuously monitored by said wet-end dewpoint
controller device 8 and said dry-end dewpoint controller device 69,
respectively. When the dewpoint within both said wet-end air/vapor
lock chamber 9 and said dry-end air/vapor lock chamber 10 is about
10% higher than the dewpoint of the atmosphere surrounding said
dryer 1--indicating an excess steam build-up within said dryer
1--either said wet-end dewpoint controller device 8 or said dry-end
dewpoint controller device 69 commands said condensation control
valve 60 to begin the operation of said internal steam-condensing
means 5 and wet-end air/vapor lock feedback control means 102 vary
the speed at which said wet-end exhaust fan 18 draws contaminant
air and said spent steam into said wet-end air/vapor lock chamber
9.
Said internal steam-condensing means 5 operates essentially as a
heat exchanger, wherein said cooling water is pumped by said
cooling water pump 55 into said wet-end drying section 53, via said
cooling water tube 62. Said cooling water enters at a temperature
of about 70.degree. F. and exchanges energy with said spent
steam--which is superheated at a temperature of about 800.degree.
F. in said wet-end drying section 53, and about 500.degree. F. in
said dry-end steam drying section 54. Said cooling water then exits
said dry-end steam drying section 54 via said cooling water tube
62, at a temperature of about 210.degree. F. As said spent steam
comes in contact with said cooling water tube 62, the temperature
of said spent steam is reduced. As a result, the specific volume of
that particular mass of steam is reduced. When the temperature of
the steam contacting said cooling water tube 62 is reduced to a
point below the dew point within any one of said drying sections
22, the steam condenses. The volume of steam to be condensed is a
function of the volume of steam exiting said steam dryer 1 via said
wet-end air/vapor lock chamber 9 and said dry-end air/vapor lock
chamber 10. As the dewpoints within said chambers 9 and 10 are
monitored, the volume of cooling water flowing through said cooling
water tube 62 is varied to provide just enough cooling to control
the volume of steam being condensed and the volume of steam exiting
said steam dryer 1. When the dewpoints within said chambers 9 and
10 both exceed the dewpoint of the atmosphere surrounding said
steam dryer 1 by less than 5%, the volume of cooling water flowing
through said cooling water tube 62 is reduced, and dry-end
air/vapor lock feedback control means 103 vary the speed at which
said dry-end exhaust fan 73 draws contaminant air and said spent
steam into said dry-end air/vapor lock chamber 10.
As previously indicated, said products 100 move into said dry-end
steam drying section 54 at a lower moisture content and, therefore,
at a lower weight than when they entered said wet-end drying
section 53. For this reason, said plug fans 13 towards the dry-end
of said steam dryer 1 operate at reduced circulation rates. Also,
as said products 100 approach the dry-end of said steam dryer 1,
they are at or near the temperature of the steam--or about
800.degree. F. In an air-rich environment this temperature would
result in spontaneous ignition of said products 100, but in the
air-free environment of the present invention spontaneous ignition
will not occur and the high temperatures increase the enthalpic
capacity of the drying steam, thereby increasing drying efficiency.
When said products have been dried to a moisture content of about
8%, they exit said dryer 1 via said exit ports 71, and enter the
next phase of the manufacturing process. If they were to leave at
the operating temperature of 800.degree. F. or more, they would
immediately explode into flame. For this reason, said indirect
heater 19 in said dry-end steam drying section 54 operates at a
temperature of about 200.degree. F. The combination of the reduced
temperature of said indirect heater 19 and the cooling effect of
the condensation process reduces the temperature of said products
100 within said dry-end steam drying section 54--that is, as they
enter said dry-end air/vapor lock chamber 10--to about 350.degree.
F.; however, they may be as much as 500.degree. F. In order to
ensure that the temperature of said products 100 exiting said
dry-end air/vapor lock chamber 10 is below their ignition
temperature, a cooling water spray device 120 controlled by a
water-coolant controller 121 operates to spray said products 100 as
necessary to reduce the product temperature at said dry-end exit
port 71. Specifically, when internal temperature measurement means
122 relays to said water-coolant controller 121 an internal dryer
temperature in excess of 375.degree. F., said water-coolant
controller 121 initiates the cooling procedure within said dry-end
air/vapor lock chamber 10. Water at a temperature of about
70.degree. F. is directed into a product-coolant pipe 123, through
a plurality of product-coolant nozzles 124 and onto said products
100, as illustrated in FIG. 11. As a result, said products exit
said dry-end air/vapor lock chamber 10 at a temperature less than
300.degree. F. Below that temperature, said products will not
ignite in air and, in addition, may be handled by automatic
stacking devices beyond said steam dryer 1.
An additional feature of said steam dryer 1--a feature which is not
essential to the actual operation of said steam dryer 1, but
provides the user with greater convenience--is said cooldown
exhaust stack 75. Said cooldown cxhaust stack 75 permits rapid
cooling within said dryer 1 so that any maintenance procedures may
be conducted shortly after the completion of a production run, or
shortly after a problem is detected within any one of said dryer
sections 22. Said maintenance access doors 77 permit entrance to
said drying sections 22 via said steam-supply shaft 30. Although a
central exhaust stack is necessary in the operation of hot-air
dryers, the present invention operates essentially as a closed
system and therefore does not require the type of large air-volume
exchanges provided by such central exhaust stacks.
Although the preferred embodiment of the apparatus and method of
the present invention has been described herein, the above
description is merely illustrative. Accordingly, it is to be
understood that the present invention is not limited to that
precisely described herein.
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